A “slit lamp” is understood to be a device that is used to observe the front portion of the eye (lens and proximal vitreous body), and to monitor contact lens fittings. The slit image facilitates diagnostic and surgical procedures. During operations, it is used as a contrast-enhancing illumination in the region of the retina.
Slit lamps of this kind are very well known, and are manufactured by several suppliers and used routinely by doctors.
The existing art includes slit lamps in which a slit aperture is illuminated, directly or via a light guide, by means of a light-emitting means. DE-C2-42 27 942, for example, describes an arrangement of a slit lamp with a light guide:
“Slit lamp illumination device for imaging a slit aperture into a target plane, comprising
                a light source, preferably of high intensity, after which is arranged a fiber-optic light guide;        a slit aperture arranged, without interposed optical elements, at a short distance after the light guide entry surface; and        a slit aperture arranged, without interposed optical elements, at a short distance after the light guide exit surface; and        an imaging device arranged after the slit aperture and having at least two optical systems each having a converging optical effect . . . ”        
In the aforesaid embodiment, a slit aperture is arranged directly after the light guide exit surface. The light guide exit surface, however, has a round cross section. A portion of the light emerging from the light guide illuminates the slit in the plane of the slit aperture. This slit is imaged by an imaging optic onto the patient's eye. A substantially larger portion of the light, however, is absorbed on the mechanical slit aperture and converted into heat, and thus does not contribute to the intensive illumination of the slit image onto the patient's eye.
This conventional embodiment thus creates the problem that because of the transition from the circular surface (exit surface of the light guide) to the rectangular surface (entrance surface of the slit aperture), considerable portions of the usable light from the light guide or from any other light-emitting means are lost.
This problem is not unknown, but the attempts to solve it, in order to eliminate the drawbacks arising therefrom, are insufficient. For example, a cylindrical lens is incorporated between the exit surface of the light-emitting means and the slit aperture. As a result, the illumination of the slit is no longer round, but instead is approximately oval or corresponds as much as possible to the slit shape. On the one hand, however, this action does not appreciably prevent the light loss, and on the other hand a cylindrical lens makes the light distribution in the slit inhomogeneous.
The inventor herein has recognized the problems of the approaches indicated above that attempt to increase the usable portion of the light. He has attempted, by way of the concept of the present invention, to achieve the most efficient possible light utilization in the slit image, while at the same time preventing absorption of the usable light on the slit aperture.